This invention relates to a method for growing single crystal alloys having two or more organic or inorganic components. More particularly, this invention concerns itself with a method for growing high purity, compositionally ungraded, single crystals of solid solutions of materials systems composed of two or more components by utilizing the well known Czochralski, Kyropoulos, Bridgmen or other similarly related melt growth techniques.
In the Bridgman method, the melt is positioned in a vertical cylindrical container which tapers to a point at the bottom of the cylinder. The cylinder is then lowered into a cold zone. The Czochralski, or so-called crystal pulling technique, involves the use of a seed crystal which is placed on the end of a rod and then lowered into a crucible containing a molten bath of crystal growing material. The seed crystal is placed in contact with the surface of molten material and then slowly withdrawn from the melt. Since freezing occurs at the interface, growth occurs on the seed as it is withdrawn, resulting in the formation of a rod-like crystal boule. The Kyropoulos method is similar to that of Czochralski. In this technique, the seed, or cooled rod, remains in contact with the molten crystal growing material and slow cooling causes crystallization to occur from the seed or rod into the melt. In these methods, the seed or rod is often rotated as a modification of the basic techniques.
However, the lack of homogenity throughout the molten solution that comes in contact with the seed, as well as the possibility of contamination from the use of a crucible containing foreign material and the necessity to replenish expanded material when growing multi-component crystal alloys, has spawned the growth of single crystal alloys of relatively poor quality with a rather high incidence of contaminated crystals lacking the high degree of purity required for many electronic applications. The problem is especially acute when growing single crystal alloys formulated from two or more compounding constituents.
The single crystal alloys grown by the method of this invention are compositionally ungraded of very high purity and, therefore, have proven to be especially useful in a wide variety of electronic applications. For example, the method of this invention is capable of producing compositionally ungraded crystal alloys from the germanium-silicon alloy system; as well as the binary, ternary or quaternary alloy systems of the Group III-V and the Group II-VI elements. The physical, electrical, optical, magnetic and chemical properties of these crystal alloys can differ from those of the components of the system and the manner in which these properties differ depends on the property and the alloy system being considered. It can be linear as in the case, for example, of the lattice constant for ternary intermetallic Group III-V compounds or, in the alternative, the variation can be from linear as, for example, the energy cap in the Ge-Si alloy system. The absolute value of a given property can be greater or less than that of any of the components of the system such as the melting point of solid solutions in systems having a continuous series of solid solutions and whose phase diagram exhibits a maximum or minimum melting point as exemplified, respectively, by the c.sup.1 and 1-carvisome and the copper-gold systems. A large number of such properties vary as a function of the solid composition and it is the ability to grow compositionally, ungraded, high quality, single crystals, tailored to provide the desired values of these properties, which makes this invention so useful.
The Ge-Si alloys, for example, are useful for forward looking infra red detector (FLIR) systems operating in the 8-14 .mu.m regions. Such systems would be able to operate at higher temperatures, thus reducing the weight and cost of such systems, in airborne reconnaissance systems remotely piloted vehicle (RPV) reconnaissance systems, and missile seeker systems. They are also useful for improved sensitivity of 1.06 .mu.m PIN detectors currently being used for laser guidance in military weather systems; and, also, as an alternative material to the use of Group III-V compounds as a detector for optical communication systems in the 1.25 .mu.m system.
The Group II-VI compounds, such as the ternary alloy, HgCdTe, are useful for infrared source and detector systems for missile guidance, for control or observation systems. The Group III-V ternary or quaternary alloys are useful for laser LED or detector systems optimized for command control and communication systems; and for, FET's, IMPATT or Gunn devices for microwave or mm wave systems.
Heretofore, a significant problem was encountered when resorting to the well known melt growth techniques, such as those of Czochralski, Kyropoulos or Bridgman. In utilizing these methods, the growing crystal was characterized by having a graded compositional content as the melt solidified. This problem could be avoided only by replenishing the melt with one of the components of the melt. Replenishing the melt, however, often resulted in the introduction of contaminants which produced crystals of lower purity than desired. With the present invention, however, the problem of a changing melt which results from the growth of a solid material of different composition from the melt from which it grows, is overcome. In the present invention, the crucible which holds the crystal growing melt is fabricated from one of the components which forms the desired single crystal alloy. This provides an in-situ source of material being depleted from the melt as the crystal is grown and a method of incorporating that component in the melt.
The technique of this invention solves the problem by producing crystals in high purity. It eliminates contamination caused by the use of an outside replenshing material, as well as contamination caused by the use of a crucible made of a foreign material. It also solves the problem of spurious nucleation caused by depletion from the melt of particles of the component being added to the melt as a solid. Such nucleation is possible if the added component is less dense than the melt since it could float to the solid liquid interface of the growing crystal before dissolving. If the added component is more dense than the melt, then it often fails to sink because of surface tension forces.